1. Field of the Invention
This invention relates to ultrasonic transducers of the type used in the pulse-echo mode of ultrasonic investigation.
2. Description of the Prior Art
Copending patent application number U.S. Ser. No. 08/037,457 to which reference is now made, describes the composition, properties and operation of an acoustoelectric ultrasonic transducer element, particularly a ZnO single crystal element. One important feature of this element is that the frequency of the electrical signal output by the transducer is in principle different from the frequency of the ultrasonic pulse which causes the electrical signal. Therefore, no acoustic separation layer is required in order to prevent interference between the transmitted ultrasonic pulse and output electrical pulse corresponding to the received ultrasonic pulse.
Another important advantage is the phase-insensitivity of the acoustoelectric element. The acoustoelectric transducer can detect a boundary having a rough or wavy surface, a boundary between materials having close acoustic impedance, a boundary between organs of a living body and so on, since phase-insensitive transducers can detect even spatially inhomogeneous waves or frequency modulated waves.
The acoustoelectric transducer also has the capability of working as a phase-sensitive transducer, i.e. using the conventional piezoelectric effect. In conjunction with this phase-sensitive feature, the acoustoelectric transducer acting as a phase-insensitive transducer and as a phase-sensitive transducer can detect energy of incident ultrasonic wave (acoustoelectric signal) and the phase of the wave (piezoelectric signal) at the same time. This feature allows the transducer to have improved sensitivity and improved S/N ratio. More advanced post-signal-processing can be employed. A good example of this feature is its application to the conventional ultrasonic micrograph technique using piezoelectric transducers. This technique utilizes energy spectral analysis; the received signals are first low pass filtered and digitized, and then their energy spectra are computed by algorithms such as FFT (fast Fourier transform). These spectra have been shown empirically as well as analytically to be closely related to the geometry and orientation of the ultrasonic reflectors such as flaws. Incident energy data in conjunction with phase data are essential information for this type of computation. The received signal is a product of intensity and phase, but a piezoelectric transducer cannot separate these. The acoustoelectric transducer can achieve this separation, as explained above.
Another advantage is that since the acoustoelectric element does not need to receive the incident ultrasonic wave perpendicularly, the angular setting of the receiving element is not critical and an adjustment mechanism is unnecessary.
U.S. Pat. No. 4,195,244 describes use of a CdS single crystal as an ultrasonic phase-insensitive acoustoelectric transducer, and suggests very briefly that this acoustoelectric transducer may be used in combination with a conventional transducer in a concentric configuration or a transmission through configuration. No details are given.
JP-A-58-63300 describes a multi-frequency ultrasonic oscillator in which two piezoelectric oscillators are laminated together, with electrodes on the outer faces and an electrode sandwiched between them. Resonances of different frequencies can be obtained, by varying the applied frequency and the method of driving. The aim appears to be to allow frequent switching of frequency, e.g. in a fish detector.